Gold (Au) bonding on aluminum (Al) pads have traditionally been used as interconnects. Aluminum pad metallizations have good self-passivation properties. However, with respect to high temperature requirements, e.g. temperatures larger than 150° C., the interfaces to gold may be limited, e.g. in terms of reliability, due to reasons such as metallic phase formation and Kirkendall voiding. There are two main approaches pursued by the semiconductor industry to overcome these reliability limitations and to avoid the costly alternative of implementing an Au pad metallization.
First, copper (Cu) wire may be applied on Al pads thereby possibly increasing life time significantly if the mechanical impact of a wire bond process is under control.
Second, pad enforcement over pad metallization (OPM) may be carried out, e.g. by electroless deposition, for example of Nickel-Phosphorus (NiP) or Palladium (Pd) or Gold (Au).
As shown in FIG. 1, an OPM 102, e.g. a palladium OPM, may be applied, e.g. over a metal pad 104, e.g. an Al pad. OPM 102 may form a reliable interface for Au wire bonding but may also be used for aluminum or copper interconnects. An optional Au-flash as pad finish may increase pad robustness against oxidation. A benefit of electroless plating for OPM is metal growth may be carried out only on the pad metallization 104, e.g. on the redistribution layer (RDL). As there may be no reaction with isolating materials 106 such as polyimide or passivation layers, consequently, there may be no adhesion of OPM to sidewalls 108. After plating, e.g. electroless plating, there may be an infinitely small gap 112 along these sidewalls which might be acceptable. However, when heat treatment is applied, e.g. during data retention bake, the amorphous deposited NiP layer may crystallize at approximately 350° C. and may cause a shrink of the NiP layer, i.e. the OPM layer, and/or a change in the shape of the imide, which may enlarge the gap at sidewalls. The enlarged broader gap 112 opens a pathway for subsequent process steps, e.g. Au-flash, which may contact the interface of the metal pad, e.g. the Al pad and NiP layer. Subsequently these interfaces may be attacked as the gap between the OPM and imide/passivation may enable corrosive attack. Furthermore, for open cavity packages, e.g. packaging processes for an isolated chip, a chip may be covered by a gel, e.g. an encapsulation material, and a gap may result in corrosion under hazardous environments. Alternative OPM arrangements are shown in FIG. 2 and FIG. 3.